THE SPONGES 



stellate spicules scattered in the flesh. A well-developed crust is frequently present ; 

 and in Geodia the crust is composed of solid siliceous globules packed into a layer, 

 beneath which lie the anchors with the prongs next the crust, and the long pointed 

 shafts passing in centripetally. The four-rayed sponges are divided into the groups 

 Choristida and Lithistida. In the former the spicules are loose and separate; and in 

 the latter desma spicules are present, and usually fused or interlocked so as to form 

 a stony skeleton. The Choristida frequently form yellowish white, leathery, nodu- 

 lated cakes, plates, and crusts. The Lithistida or stony sponges are usually brittle 

 and friable, or ofsstone-like hardness. The four-rayed sponges belong mostly to 

 shallow water, but a few specimens have been obtained from depths of nearly two 

 thousand fathoms. 



THE FLESHY SPONGES Order CARNOSA 



These form a small group of uncertain systematic position, their chief features 

 being the possession of a tough rind, inclosing a softer pith, the absence or slight 

 development of a skeleton, and the highly developed canal system. They appear 

 to be related to the four-rayed sponges. The soft pith contains the flagellated 

 chambers and the canals leading to and from them. The genus Chondrilla possesses 

 isolated siliceous spiny spheres, especially situated along the courses of the canals 



SEA KIDNEY LEATHER SPONGE (Chondrosia reniformis), a. Specimen cut open. 



and beneath the rind. The allied Chondrosia of the Mediterranean takes the form 

 of leathery knobs or cakes with a slimy surface. The usually solitary oscule is 

 irritable and contracts slowly when the sponge is taken from the water. Fisher- 

 men call this sponge, sea flesh or sea kidney. The ground substance contains no 

 skeleton of silica or horny material; and the in-current and out-current canals form 

 two sets of tree-like branched systems with the flagellated chambers interpolated 

 between the final twigs of each. 



SINGLE-RAYED 'SPONGES Order MONAXONIDA 



These sponges are those most frequently met with on the British shores and 

 in shallow water .throughout the world. The skeleton is mainly built up of 



